To my knowledge, early firearms actually lead to armour becoming thicker and more angled rather than disappearing. Those round little pistol balls didn't penetrate solid steel as well as one may think. Then again, flintlocks were pretty advanced compared to the earliest firearms, so maybe you had something more effective in mind.

Anyway, I don't give a rat's ass about realism, so if you want bullets to instant-kill dragons, or if you want powerful enough warriors to be able to deflect them with their teeth, I don't care either way. I like that you have some idea of where to take this story, but I'll wait until you're sure what the setting's going to be like.